It has been known that a diamond semiconductor has a rather large bandgap, such as approximately 5.5 eV (corresponding to a light wavelength of approximately 225 nm), at room temperature, and that in an intrinsic state in which a dopant (impurity) is not added, it behaves as an insulating material. As a method for growing a single crystal thin film, a microwave-excited plasma vapor growth method (Patent Document 1) which is performed in an atmosphere substantially containing carbon and hydrogen, such as CH4 (methane) and H2 (hydrogen) gases, has been developed and widely used. In addition, in the microwave-excited plasma vapor growth method, a method for controlling a p-type (primary carrier is a hole) electrical conduction property by addition of B (boron) as a dopant has also been widely used.
Since the microwave-excited plasma vapor growth method is a vapor growth method using an atmosphere containing hydrogen, it has been known that a grown diamond single crystal film surface is a surface substantially covered with hydrogen. That is, it has been known that a C—H molecular structure (hereinafter referred as “hydrogenation”) in which dangling bonds of carbon (C) atoms are terminated by hydrogen (H) atoms by bonding is present on the surface, and that by this hydrogenation, in the diamond in the vicinity of the surface thereof, a surface conductive layer is generated, that is, holes, which are primary carriers, are locally present in the vicinity of the surface (within 2 nm). It has also been known that this surface conductive layer is also present in undoped and boron doped (100) and (111) plane single crystal thin films and a polycrystalline thin film.
The generation mechanism of this surface conductive layer has caused a great controversy on a global basis; however, at least from an experimental point of view, it has already been known that the surface conductive layer (1) is stably present up to approximately 200° C., and (2) is generated only at a hydrogenated diamond surface. It has also been known that by a solution treatment (oxidation treatment) removing surface-bonded hydrogen, for example, by a treatment of immersion in a mixed solution of boiled sulfuric acid and nitric acid, this surface conductive layer disappears, and the inventors of the present invention also confirmed this fact.
As a light sensing device which detects ultraviolet light irradiated to a light-receiving unit by the change in electrical resistance thereof or the change in amount of light induced current, heretofore, there have been conceived, for example, devices using, as a solid material of the light-receiving unit, a Si semiconductor having a detection sensitivity also to visible light and the like in a wavelength of 400 to 650 nm, and an AlxGa1-xN (0≦x≦1) semiconductor and a diamond semiconductor, which has no detection sensitivity to the above visible light and the like and to noise light in an infrared region.
A light detection principle of these light sensing devices is to detect the change in electrical resistance or the change in amount of light induced current caused by carriers of electron-hole pairs which are generated in a semiconductor by irradiation of light having an energy more than a bandgap to the semiconductor in a light-receiving unit. Hence, a device structure can be formed in accordance with a two-terminal device composed of a semiconductor and two electrodes bonded thereto, and as a result, an extremely simplified ultraviolet sensor can be manufactured.
As the light sensing device made of a two-terminal device, there are widely used a metal-semiconductor-metal structure (MSM) device having a comb type electrode structure, and a Schottky type device which has two different type electrodes, i.e., a rectifier electrode and an ohmic electrode, and which detects light through the rectifier electrode.
As an example in which a diamond semiconductor is used for an ultraviolet sensing device, for example, in Non-Patent Document 1, among MSM type photoconductive sensing devices, each of which uses a surface conductive layer of a polycrystalline diamond thin film in a light-receiving unit and each of which uses Ti and Au for a first layer electrode and a second layer electrode, respectively, a device has been disclosed that achieves a detection sensitivity of 0.03 A/W to irradiation of ultraviolet light having a wavelength of 200 nm. In addition, in Non-Patent Document 2, among MSM type photoconductive sensing devices, each of which uses a polycrystalline diamond film in a light-receiving unit, a surface conductive layer of the diamond film being removed by an oxidation treatment, and each of which uses Ti and Au for a first layer electrode and a second layer electrode, respectively, a device has been disclosed that achieves a detection sensitivity of 0.02 A/W to irradiation of ultraviolet light having a wavelength of 200 nm. In addition, in Non-Patent Document 3, among Schottky type sensing devices in each of which a rectifier electrode of Au and an ohmic electrode of Ti/Ag/Au (in this case, “/” indicates the order of deposition) are formed on a polycrystalline diamond thin film, although the detection sensitivity is not known, a device has been disclosed that has a visible light blind ratio of five orders of magnitude, which is obtained between irradiation of light having a wavelength of 200 nm and that of light having a wavelength of 600 nm.
In addition, as an example of a prior art, in Patent Document 2, a technique has been disclosed relating to a diamond ultraviolet sensing device which uses in a light-receiving unit a diamond polycrystalline thin film or (100) and (111) oriented thin films, each having a thickness of 40 μm, and a surface from which bonded hydrogen is removed; however, this device has insufficient detection sensitivity for practical use. In Patent Document 3, a diamond ultraviolet sensing device has been disclosed which uses a surface conductive layer of diamond in a light-receiving unit. However, the detection sensing wavelength of this device covers the entire visible light region, this device is a photoconductive type sensing device using a defect level in the bandgap of diamond, and hence ultraviolet light having a wavelength of 250 nm or less cannot be selectively detected.    Non-Patent Document 1: J. Looi, M. D. Whitfield, and R. B. Jackman, Appl. Phys. Letts. 74, 3332 (1999)    Non-Patent Document 2: R. D. McKeag and R. B. Jackman, Diamond Relat. Mater. 7, 513 (1998)    Non-Patent Document 3: M. D. Whitfield, S. S M. Chan, and R. B. Jackman, Appl. Phys. Letts. 68, 290 (1996)
Patent Document 1: Japanese Examined Patent Application Publication No. 59-27754
Patent Document 2: Japanese Unexamined Patent Application Publication No. 11-248531
Patent Document 3: Japanese Unexamined Patent Application Publication No. 11-097721